The present invention relates to methods and apparatus for detecting a pretilt angle of an element in which the direction of orientation of molecules is twisted between a light incident side and a light outgoing side.
Liquid-crystal display devices using liquid-crystal cells have been widely used in monitors and displays due to their characteristics, such as low weight, small thickness, and low energy consumption. The display principle of liquid-crystal cells is based upon the fact that the direction of orientation of liquid crystal molecules changes and the polarization state of the incident light on the liquid-crystal cell is modulated when a voltage is applied across the liquid-crystal cell.
Various types of liquid-crystal cells utilized in liquid-crystal displays are known. Twisted nematic liquid-crystal cells (herein referred to as xe2x80x9cTN liquid-crystal cellsxe2x80x9d) and super twisted nematic liquid-crystal cells (herein referred to as xe2x80x9cSTN liquid-crystal cellsxe2x80x9d) have been widely used.
TN liquid-crystal cells are cells in which the liquid-crystal molecules are oriented parallel to the substrate surface, but the orientation direction is twisted through an angle of almost 90xc2x0 between two substrates. STN liquid-crystal cells arc cells in which the orientation direction of the liquid-crystal molecules is twisted through an angle of no less than 90xc2x0 (for example, about 180xc2x0xcx9c270xc2x0) between two substrates. Usually, TN liquid-crystal cells and STN liquid-crystal cells are provided with an initial orientation by tilting the liquid crystal molecules with respect to the substrate surface in order to improve display performance. The angle of such initial orientation is called a pretilt angle.
Because the display performance of liquid crystal cells changes depending upon the pretilt angle, it is necessary to detect the pretilt angle in order to design liquid-crystal cells and control the manufacturing process. Furthermore, the pretilt angle also should be determined when an orientation technology, which requires the initial orientation of liquid crystal molecules to be a desired pretilt angle, is developed or when the relationship between display performance and the pretilt angle of liquid crystal molecules is being studied.
A crystal rotation (CR) method is known as a method for detecting the pretilt angle (J. Appl. Phys., Vol. 48, No 1783, 1977). According to the crystal rotation method, a twist-free, horizontally-oriented liquid-crystal cell is disposed between a polarizer and an analyzer and the transmitted light intensity is measured while changing the incident angle of the incident light on the liquid-crystal cell. The pretilt angle is then calculated by using incident angles that correspond to a maximum or minimum intensity of the transmitted light. Methods for detecting the pretilt angle that have utilized the improved crystal rotation methods were disclosed in Japanese Laid-open Patent Application Nos. 8-94445 and 11-160198.
However, the crystal rotation method is basically a method for detecting a pretilt angle of a liquid-crystal cell in which the orientation direction is not twisted. Therefore, it is not suitable for detecting the pretilt angle of liquid-crystal cells having a twisted orientation direction, such as TN liquid-crystal cells and STN liquid-crystal cells.
Accordingly, several pretilt angle detection methods have been proposed that are suitable for detecting the pretilt angle of liquid-crystal cells having a twisted orientation direction.
Thus, Japanese Laid-open Patent Application No. 5-18860 disclosed a method for detecting a pretilt angle of a liquid-crystal cell by using a photoelastic modulation element (PEM) and analyzing the polarization state of the light outgoing from the liquid-crystal cell.
Japanese Laid-open Patent Application No. 6-74864 disclosed a method for measuring the intensity of light that has passed through a tilted liquid-crystal cell (transmitted light intensity) and determining the pretilt angle of the liquid-crystal cell based upon the inclination angle at which the transmitted light intensity reaches a maximum.
Japanese Laid-open Patent Application No. 9-152321 disclosed a method for measuring spectral characteristics of light that has passed through a tilted liquid-crystal cell (transmitted light intensity) and determining the pretilt angle of the liquid-crystal cell based upon the spectral characteristics of the transmitted light.
Japanese Laid-open Patent Application No. 11-352449 disclosed a method for measuring the transmitted light intensity in a liquid-crystal cell by using light having a plurality of wavelengths and determining the pretilt angle of the liquid-crystal cell based upon the measured intensity of the transmitted light.
The method disclosed in Japanese Laid-open Patent Application No. 5-18860 is based upon the assumption that the thickness d of the liquid crystal cell, the twist angle "PHgr" of orientation between two substrates, and the refractive indexes no, ne of the liquid-crystal material with respect to ordinary and extraordinary light are known in advance. In the case of rod-like liquid crystal molecules, ordinary light is polarized in the direction perpendicular to the longitudinal axis of the liquid crystal molecule (electric flux density oscillates in this direction), and the extraordinary light is polarized in the direction perpendicular to the ordinary light (electric flux density oscillates in the direction perpendicular to the oscillation direction of the electric flux density for ordinary light). However, the thickness d of the liquid-crystal cell and the twist angle "PHgr" of orientation are, in fact, not known in advance and should be measured separately. Moreover, even if those parameters are determined, the value obtained is different from the actual pretilt angle. Furthermore, an expensive photoelastic modulation element (PEM) is necessary for these measurements.
Japanese Laid-open Patent Application No. 6-74864 does not disclose a specific method for calculating a pretilt angle of a liquid crystal cell based upon the tilt angle of the liquid-crystal cell at which the transmitted light intensity reaches a maximum. By altering the tilt angle of the liquid-crystal cell, the transmitted light intensity changes. However, the transmitted light intensity greatly depends upon the thickness d of the liquid-crystal layer, the twist angle "PHgr" of orientation between the substrates, the refractive indexes no, ne of the liquid-crystal material with respect to ordinary and extraordinary light, and the wavelength xcex of the light utilized for the measurements. For this reason, the relationship between the pretilt angle and tilt angle of the liquid-crystal cell at which the transmitted light intensity reaches a maximum is not simple.
According to the method disclosed in Japanese Laid-open Patent Application No. 9-152321, the transmitted light intensity must be calculated using a 4xc3x974 matrix method for polychromatic light each time that changes are made to the thickness d of the liquid-crystal layer, the twist angle "PHgr" of orientation between the substrates, the refractive indexes no, ne of the liquid-crystal material with respect to ordinary and extraordinary light, and the wavelength xcex of light utilized for the measurements. Therefore, the amount of calculations is enormous. Furthermore, because the measurements must be performed with polychromatic light, the light source and the photodetector become complex.
According to the method disclosed in Japanese Laid-open Patent Application No. 11-352449, the measurements must be performed using polychromatic light and the transmitted light intensity must be calculated using a 4xc3x974 matrix method for the polychromatic light. Therefore, the same problems are encountered as those described with reference to the method disclosed in Japanese Laid-open Patent Application No. 9-152321. Furthermore, the specification of Japanese Laid-open Patent Application No. 11-352449 does not disclose a specific method for calculating the pretilt angle.
It is an object of the present invention to provide pretilt angle detecting methods and apparatus capable of determining a pretilt angle of an element, in which the orientation direction of molecules is twisted between the light incident side and light outgoing side, within a short time and in a simple manner by using a simple measurement apparatus.
In accordance with a first aspect of the present invention, the transmitted light intensity is measured by changing the light incident angle upon a sample having a twisted orientation and the pretilt angle of the sample having a twisted orientation is determined by analyzing the dependence of the transmitted light intensity on the light incident angle for a plurality of light incident angles.
In accordance with another aspect of the present invention, the light incident angle on a sample having a twisted orientation is changed, a plurality of transmitted light intensities is measured for each light incident angle, and the pretilt angle of the sample having a twisted orientation is determined by analyzing the dependence of the transmitted light intensity on the light incident angle for a plurality of light incident angles.
In accordance with the present invention, it is not necessary to use a plurality of lights having different wavelengths. Therefore, the pretilt angle can be determined within a short time by using a simple measurement apparatus.
In a preferred embodiment of the present invention, Stokes parameters for a plurality of light incident angles are determined based upon the transmitted light intensity measured for a plurality of light incident angles, and then the pretilt angle of the sample having a twisted orientation is determined based upon the Stokes parameters. When the pretilt angle is determined by using the Stokes parameters, a simpler measurement apparatus can be used and the measurement time can be further shortened.
In another preferred embodiment of the present invention, an apparent retardation for a plurality of light incident angles is determined based upon the measured transmitted light intensity for a plurality of light incident angles, and then the pretilt angle of the element is determined based upon the apparent retardation When the pretilt angle is determined by using the apparent retardation, complex simulation becomes unnecessary. Therefore, the amount of calculations is greatly reduced.
In still another preferred embodiment of the present invention, Stokes parameters for a plurality of light incident angles are determined based upon the measured transmitted light intensity for a plurality of light incident angles, the apparent retardation is determined based upon the Stokes parameters, and the pretilt angle of the element is determined based upon the apparent retardation. In this case, the average pretilt angle is determined based upon the apparent retardation and the pretilt angle can be determined based upon the average pretilt angle.
In still another preferred embodiment of the present invention, the transmitted light intensity is measured in a state in which the following relationship is satisfied between the orientation direction xcex1in(rad) of molecules at the light incident side interface of the sample having a twisted orientation and the twist angle "PHgr" (rad) of the element;       tan    ⁢          xe2x80x83        ⁢          α              i        ⁢                  xe2x80x83                ⁢        n              =      -                            Φ          -                      sin            ⁢                          xe2x80x83                        ⁢            Φ                                    1          -                      cos            ⁢                          xe2x80x83                        ⁢            Φ                              .      
The pretilt angle can be determined accurately by measuring the transmitted light intensity in such arrangement.
The term xe2x80x9ca plurality of optical clement arrangementsxe2x80x9d refers to a state in which the structural elements of an optical system or arrangement angles of structural elements differ, such as, e.g., in the case when a quarter-wave plate is inserted, and the analyzer is rotated through a prescribed angle.
The objects and advantages of the present invention can be more clearly understood from the description of the embodiments described below or the claims, as illustrated by the drawings.